Ah, yes...but study the actual sounds.

"Pinnnn....nnnumber"

The first word ends in the "n" sound. The second word begins with the "n" sound. This makes it more comfortable (and economical) for the human tongue and palate to say "PIN number" than to say "P.I. number".

Try it and see. ;-) The first way has only 3 syllables and flows effortlessly. The 2nd way has 4 syllables and has a more awkward and broken up sonic flow. I think that's why people like to say "PIN number", because it's easier to say.

Now to say just "Pin" and not say "number at all", that's easy enough for sure, but people want to emphasize the mental concept of the "number", so it's not as satfisfying for them to just say "Pin"...which might after all be misunderstood by the listener to mean something else entirely, if the listener wasn't paying attention too closely (and that would describe about 95% of the population at any given time).